Within the terms of the present invention, a luminescent feature substance is a substance consisting of one single component or of a mixture of a plurality of components, which show a luminescence behavior. These feature substances, which e.g. can be present in the form of pigments, are contained in the document of value itself and/or are applied thereon. Such documents of value can be e.g. bank notes, checks, chip cards, ID documents, passports or the like.
There exists a number of known systems for checking such documents of value. One system is described in the DE 23 66 274 C 2 of the applicant. In this system a bank note is inserted in a checking device, is irradiated at certain points and the remitted fluorescence radiation is captured in a spectrally resolved manner, in order to determine, whether a fluorescent authenticity feature is actually present in the bank note to be checked. The feature substances detected with such an apparatus usually are located in a defined spatially limited area of the bank note.
From the WO 01/48311 A2 of the applicant is further known, that for differentiating different nominal values of a currency system the bank notes have a coding, in which as luminescent feature substances mottled fibers are incorporated in areas of the bank note surface separated from each other. In this case the coding is represented by a defined different geometric arrangement of these partial areas or by the type or presence or absence of the mottled fibers in these partial areas.
For measuring the mottled fibers incorporated in the known discrete areas of the bank note surface, the bank notes are separately scanned along a measuring track extending in transport direction, in order to determine the type or presence or absence of mottled fibers in the individual areas of the measuring track and their distance to each other. With that the coding of the checked bank note can be determined.
But it has turned out, that these known systems have a low measuring sensitivity, in particular with feature substances having a low luminescence intensity that are incorporated in the document of value in a not locally concentrated fashion, but distributed over a large surface.